Spin-based devices offer non-volatility and compact area compared to complementary metal oxide semiconductor (CMOS) devices. One such spin-based device is the magnetoelectric magnetic tunnel junction device (MEMTJ), which can be thought of as a three terminal beyond-CMOS device with logic and memory capabilities.
The MEMTJ device can be fabricated at the back-end-of-line (BEOL) of the CMOS process between metal layers in a similar manner to that of the spin transfer torque random access memory (STT MRAM), making it compatible with the CMOS process. FIG. 1 shows a MEMTJ device structure 100 that includes at least two ferromagnetic layers: a fixed ferromagnetic layer 120 and a free ferromagnetic layer 140. A thin tunnel dielectric Magnesium oxide (MgO, or another suitable insulator) insulator 130 separates the fixed ferromagnetic layer 120 and free ferromagnetic layer 140. The MEMTJ stack 100 also includes an antiferromagnetic substrate 110 on which the stack may be mounted and a thin chromia (Cr2O3) layer 150 integrated on top of a free ferromagnetic (FM) layer 140 below the electrode 160.
The MEMTJ structure 100 includes three CMOS-like terminals: “G” or Gate—electrode, “D” or Drain—Fixed FM layer, and “5” or source—Free FM layer. When a voltage VGS is applied across the electrode 160 and free FM layer 140, there is a resulting resistance change, thus a current change is observed across the free and the fixed FM layers 140, 120. Voltage-controlled switching of the ME layers allows switching to be achieved with low power dissipation compared to the CMOS process. The device offers the benefit of low power operation (˜aJ), nonvolatility and compact cell area compared to the CMOS process with two process nodes enhancement.
Logic circuits using the ME-MTJ devices draw on a bipolar logic voltage, i.e. positive and negative polarity of the supply voltage, each around 0.1V, and a ground supply. The supplies are required to switch the spin vector state of the magneto-electric antiferromagnetic (MEAFM) chromia spin vector orientation. The switching mechanism utilized for the switching of MTJ cell in the MEMTJ device is exchange biasing.
The MEMTJ device 100 has been used to develop circuits, including a full-adder. In use, however, the input voltage requirement is bipolar (−0.1 v to +0.1 v), while the output voltage (when the resistance across the tunnel layer 130 is converted to voltage), is Unipolar (0 v to 0.2 v). This can and has been resolved with the use of level shifters and/or reset circuitry, but it does require additional, often CMOS circuitry.